STIM1 Clusters and Activates CRAC Channels via Direct Binding of a Cytosolic Domain to Orai1

Store-operated calcium channels (SOCs) are a major pathway for calcium signaling in virtually all metozoan cells and serve a wide variety of functions ranging from gene expression, motility, and se...

Store-Operated Calcium Channels

Ca2+ entry into cells of the peripheral immune system occurs through highly Ca2+-selective channels known as CRAC (calcium release-activated calcium) channels. CRAC channels are a very well-characterized example of store-operated Ca2+ channels, so designated because they open when the endoplasmic reticulum (ER) Ca2+ store becomes depleted. Physiologically, Ca2+ is released from the ER lumen into the cytoplasm when activated receptors couple to phospholipase C and trigger production of the second messenger inositol 1,4,5-trisphosphate (IP3). IP3 binds to IP3 receptors in the ER membrane and activates Ca2+ release. The proteins STIM and ORAI were discovered through limited and genome-wide RNAi screens, respectively, performed in Drosophila cells and focused on identifying modulators of store-operated Ca2+ entry. STIM1 and STIM2 sense the depletion of ER Ca2+ stores, whereas ORAI1 is a pore subunit of the CRAC channel. In this review, we discuss selected aspects of Ca2+ signaling in cells of the immune syste...

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Molecular Basis of Calcium Signaling in Lymphocytes: STIM and ORAI

Influx of Ca(2+) through store-operated Ca(2+) channels (SOCs) is a central component of receptor-evoked Ca(2+) signals. Orai channels are SOCs that are gated by STIM1, a Ca(2+) sensor located in the ER but how it gates and regulates the Orai channels is unknown. Here, we report the molecular basis for gating of Orais by STIM1. All Orai channels are fully activated by the conserved STIM1 amino acid fragment 344-442, which we termed SOAR (the STIM1 Orai activating region). SOAR acts in combination with STIM1 (450-485) to regulate the strength of interaction with Orai1. Activation of Orai1 by SOAR recapitulates all the kinetic properties of Orai1 activation by STIM1. However, mutations of STIM1 within SOAR prevent activation of Orai1 but not co-clustering of STIM1 and Orai1 in response to Ca(2+) store depletion, indicating that STIM1-Orai1 co-clustering is not sufficient for Orai1 activation. An intact carboxy terminus alpha-helicial region of Orai is required for activation by SOAR. Deleting most of the Orai1 amino terminus impaired Orai1 activation by STIM1, but Orai1(Delta1-73) interacted with and was fully activated by SOAR. Accordingly, the characteristic inward rectification of Orai is mediated by an interaction between the polybasic STIM1 (672-685) and a Pro-rich region in the N terminus of Orai1. Hence, the essential properties of Orai1 function can be rationalized by interactions with discrete regions of STIM1.

SOAR and the polybasic STIM1 domains gate and regulate Orai channels

Stromal interaction molecule (STIM) proteins function in cells as dynamic coordinators of cellular calcium (Ca2+) signals Spanning the endoplasmic reticulum (ER) membrane, they sense tiny changes in the levels of Ca2+ stored within the ER lumen As ER Ca2+ is released to generate primary Ca2+ signals, STIM proteins undergo an intricate activation reaction and rapidly translocate into junctions formed between the ER and the plasma membrane There, STIM proteins tether and activate the highly Ca2+-selective Orai channels to mediate finely controlled Ca2+ signals and to homeostatically balance cellular Ca2+ Details are emerging on the remarkable organization within these STIM-induced junctional microdomains and the identification of new regulators and alternative target proteins for STIM

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STIM proteins: dynamic calcium signal transducers

Dynamic coupling of the putative coiled-coil domain of ORAI1 with STIM1 mediates ORAI1 channel activation.

Resting State Orai1 Diffuses as Homotetramer in the Plasma Membrane of Live Mammalian Cells

A combined optical and atomic force microscope for live cell investigations

Resting-State Orai1 Diffuses as Homotetramer in the Plasma Membrane of Live Mammalian Cells

Ca2+ release activated Ca2+ (CRAC) channels mediate robust Ca2+ influx when the endoplasmic reticulum Ca2+ stores are depleted. This essential process for T-cell activation as well as degranulation of mast cells involves the Ca2+ sensor STIM1, located in the endoplasmic reticulum and the Ca2+ selective Orai1 channel in the plasma membrane. Our review describes the CRAC signaling pathway, the activation of which is initiated by a drop in the endoplasmic Ca2+ level sensed by STIM1. This in term induces multimerisation and puncta-formation of STIM1 proteins is followed by their coupling to and activation of Orai channels. Consequently Ca2+ entry is triggered through the Orai pore into the cytosol with subsequent closure of the channel by Ca2+-dependent inactivation. We will portray a mechanistic view of the events coupling STIM1 to Orai activation based on their structure and biophysics.

Josef Madl

Papers

Dynamic coupling of the putative coiled-coil domain of ORAI1 with STIM1 mediates ORAI1 channel activation.

Resting State Orai1 Diffuses as Homotetramer in the Plasma Membrane of Live Mammalian Cells

A combined optical and atomic force microscope for live cell investigations

Resting-State Orai1 Diffuses as Homotetramer in the Plasma Membrane of Live Mammalian Cells

Structure, regulation and biophysics of I(CRAC), STIM/Orai1.